This study will investigate the efficacy of ibuprofen and acetaminophen in children who present at the Children's Hospital emergency room. This study will test the following hypotheses: 1) ibuprofen lowers temperature more or longer than acetaminophen mg for mg in children 6 months to 11 years. 2) ibuprofen is more potent than acetaminophen in reducing fever in children 6 months to 11 years. 3) ibuprofen is not associated with significant toxicity.